The present invention relates to a light sensor having a phototransistor and an impedance converter for a potential of a main electrode of the phototransistor.
Such a light sensor is discussed, for instance, in German Published Patent Application No. 4 299 536.
As characterized, in this light sensor, the control electrode of the phototransistor is connected, together with its second main electrode, to a first supply potential of the light sensor.
Also, as characterized, in this light sensor, the phototransistor is operated in a weak inversion condition, which gives it a linear output characteristic of the photocurrent or a logarithmic output characteristic of the photovoltage as a function of the received illuminance. Such sensors may be advantageous because they may make possible registration free from modulation over several decades, and are thereby well suited for optical detection applications, for example, in motor vehicle technology, where pictures have to be processed which have extremely strong differences of brightness, as, for instance, while driving with oncoming traffic during darkness or in case of sunlight reflections from a preceding vehicle, the correct registration of which challenges even the human eye.
A disadvantage of this sensor may be that operation in a weak inversion condition may deliver only a very low photocurrent density of the light-sensitive boundary surface of the phototransistor, and it therefore may be necessary to use large surface phototransistors to generate photocurrents of sufficient strength and having adequate signal-to-noise ratio. The high-sensitivity resolution of a light sensor constructed from an arrangement of such light sensors is correspondingly low.
A further problem that may arise from the necessary great extension of the light-sensitive boundary surfaces may be their considerable capacitance. The greater the capacitance, the longer it may take, at a change of illuminance, for the photocurrent, changed in response to the change in illumination, to lead to a change in potential at the input to the source-follower, and thus, to a change in the measuring signal delivered by the light sensor. This effect may be referred to as a xe2x80x9cstreaking effectxe2x80x9d.
However, for applications in the automobile sector, fast reaction times of light sensors may be required, because, in order to effectively support the driver of a motor vehicle while traveling, such applications, as, for example, automatic rear-end collision protection while driving in convoy, may require a shorter reaction time than the driver.
The feedback element provided according to an exemplary embodiment of the present invention may provide great changes in the output signal of the light sensor at relatively low illumination-dependent changes in the terminal potentials of the phototransistor, and, as a result, at relatively low charges to be shifted at an illumination change in the phototransistor. A high sensitivity and high reaction speed of the light sensor may simultaneously be reached in this way.
According to the exemplary embodiment of the present invention, when the second main electrode of the phototransistor and its control electrode are no longer tied directly to the supply potential, as may be the case with the light sensor discussed in the Background Information, but are instead coupled via a feedback element, the gate potential of the phototransistor may be varied as a function of photocurrent or of the brightness hitting the phototransistor, respectively. The conductivity of the phototransistor modulated thereby, at low fluctuations of the photocurrent, should lead to strong changes in potential at the first main electrode of the phototransistor which the impedance converter converts into a signal which can be further processed outside the light sensor.
In order to attain a logarithmic characteristic response of the light sensor, and thus, its applicability over more than several decades of illuminance, the phototransistor may be expediently operated in a weak inversion condition.
As a feedback element, a transistor can be used which has, for example, a control electrode connected to the second main electrode of the phototransistor, a first main electrode connected to the control electrode of the phototransistor and a second main electrode connected to a first supply potential. This transistor, too, may be operated in a weak inversion condition.
As impedance converter, a further transistor can be used, which is connected to the first main electrode of the phototransistor, as source-follower.
A resistive element, which may likewise be a transistor, can be connected between the second main electrode of the phototransistor and the first supply potential; in order to make possible changes in the potential at the control electrode of the transistor used as feedback element, as a function of the photocurrent.
A semiconductor junction of the phototransistor operated in the blocking direction forms a current path between the first and a second supply potential. This junction can be, for example, the junction between a bulk electrode of the phototransistor and its source electrode. This bulk electrode may also be connected directly to the first supply potential (VDD in FIG. 2). An additionally shortened reaction time of the light sensor may occur when the bulk electrode is connected to the output of the source-follower transistor.
An additionally strengthened feedback may be attained when the first main electrode of the transistor forming the feedback element is connected to the second main electrode of the source-follower transistor.